Thyroglobulin labeled with radioiodine will be purified, reduced and alkylated and cleaved with cyanogen bromide and subsequent digestion with trypsin. Iodinated peptides will be purified by Sephadex and ionic exchange column chromatography and possibly by high voltage or disc electrophoresis. Aminoacid analysis, identification of N and C terminal aminoacids, disc electrophoresis and peptide mapping will be performed at each step of purification. Iodoaminoacid composition will also be determined. The purified peptides will be analyzed for the aminoacid sequence by the method of Edman. Antisera against iodinated peptides will be produced in rabbits and their interaction with the intact thyroglobulin molecule as well as with its fragments will be studied. The antisera also will be coupled to Sepharose and affinity chromatography will be performed as another purification procedure. Affinity chromatography will also be used to identify the subunits or chains of thyroglobulin containing the respective iodinated peptide. The significance of the study is in correlation of the primary structure of thyroglobulin and thyroid hormone synthesis. It may also help to elucidate the tertiary structure of the protein. The results may become a tool for studying differences of the thyroglobulin structure between different animal species and different thyroid conditions in human pathology.